1. Field of the Invention
The present invention reltes to a magnetic head, and especially a magnetic head having magnetic material sandwiched on both side thereof by substrate material.
2. Description of the Prior Art
Ferrite is conventionally used as a material for the magnetic head core, because it has superior practicality and resistance to wear, but its saturation magnetization is lower by 30-50% than that of alloy materials. The saturation magnetization of the material which comprises the head core becomes a problem when ferrite is used for the head core for use with media for high density (high coercive force) recording which has appeared in recent years. A sendust or an amorphous alloy is used as the material for the core.
Amorphous alloy for magnetic head use is in the limelight because it has superior resistance to wear and superior magnetic characteristics. The amorphous alloy is suitable for making thin films less than 50 .mu.m thick because of its manufacturing characteristics. And also, metal such an amorphous alloy has much smaller specific resistance than that of ferrite. The eddy current loss of the metal in the high frequency region is larger than that of ferrite, so that an amorphous alloy is convenient. Such thin sheet films, however, are not easily machined and are not very strong. Accordingly, a magnetic head having a thin film sandwiched on both sides thereof by a substrate material having superior resistance to wear is in ordinary practical use. Here, however, the bonding of magnetic material to the substrate material is likely to be problematic. Organic adhesives are easy to handle, and easy to heat for hardening, but they must have a minimum thickness for practical use, for example, several tens of microns. Furthermore, these organic adhesive have poor reliability when subjected to changes in temperature and humidity after the bonding, so that it is difficult to maintain the shallow gap length of the magnetic head, which is used for a video tape recorder and the like.
The most reliable bonding is the bonding by melted glass.
FIG. 2 shows the relationship between magnetization .sigma. and temperature T. As shown in FIG. 2, generally the magnetization becomes negligible at Curie temperature Tc. And the magnetization increases near crystallization temperature Tx because the crystalline alloy exhibits the magnetic characteristics again.
When the magnetic characteristic of magnetic material is considered for use in a magnetic head, a higher magnetic permeability is desirable. It is necessary to reduce the anisotropy of the magnetic film so as to gain the high magnetic permeability. Accordingly, the magnetic film must be heat treated in a temperature region which is higher than Tc and lower than Tx, to reduce the taking off the anisotropy of the magnetic film.
On the other hand, there are materials wherein the order of Tc into Tx is inversed, and such materials can not have high magnetic permeability by the normal heat treatment. A special heat treatment such as heat treatment in magnetic field is necessary to gain high magnetic permeability. But such special heat treatment is a problem to practice, and so the amorphous alloy shown in FIG. 2 is normally used for a magnetic head.
When considering the manufacturing process of the magnetic head which uses amorphous alloy bonded by melted glass, at least two steps of heat treatment, namely heat treatment process for bonding substrate material and amorphous alloy, and heat treatment process for making the gap, are necessary. When considering the looseness and the movement of the glass, it is necessary to use a glass having a high softening temperature for bonding the substrate and amorphous allo and to use a glass having a low softening temperature for making the gap, is ideal. On the other hand, when considering the reduction of anisotropy of the magnetic film, the temperature of final heat treatment process of manufacturing of the magnetic head is necessary to be selected at least the same as or more than that of the heat treatment processes made before that.
Generally, the crystallization temperature Tx is generally about 500.degree. C., and the Curie temperature Tc is above 450.degree. C. when considering the magnetic flux density as the magnetic head material, in practice. Accordingly, when considering materials, at present the only one which has a narrow temperature region between Tc and Tx and a working temperature lower than Tx is lead glass. In order to decrease the softening temperature of the glass, the content of lead in the glass may be increased. However, the glass becomes unstable due to the increase in the content of lead, and its mechanical strength becomes weak. Therefore, it is impossible to decrease the softening temperature.
From above-mentioned point of view, the glasses having softening temperatures about 400.degree.-500.degree. C. are used for the low melting point glass. The glass for bonding the substrate is the same as that used to make the gap. Even when glasses of different melting points are properly used for bonding the substrate and making the gap, the substrate-bonding glass layers may become loose during the gap making process because the temperature used in the gap making process is close to their softening temperatures. Therefore, problems are created, such as low accuracy of gap length, core slippage and decrease yield. FIG. 1 schematically shows the looseness of glass layers and the like magnified. In actuality they are in the order of micron displacements. In FIG. 1, substrate 1 is bonded to magnetic material 2 by melted glass layer 3, and gap 4 is firmly maintained. Crevice 5 is, however, brought about by the looseness of the melted glass layer 3.
On the other hand, there is prior art wherein crystallized glass is applied to a gap part of a magnetic head (which is shown in Japanese patent unexamined publication Sho 55-108922. In this prior art, however, the glass composition obtained by sputtering method may be shifted from its original composition and sufficient bonding hardness can not be assured on rough surfaces, except for the gap surface, because of the thinness of the glass film layer.